Telephone answering and call forwarding improvement

ABSTRACT

An improvement to automatic answering and call forwarding features used in telephone communications which allows a user to remotely activate, modify, and deactivate these features. The improvement describes a transformerless means for coupling signals to and from the telephone line without compromising the operation of the telephone line. The apparatus employs a microprocessor controller (11) with memory (12), a tone encoder (15), a tone decoder (14), a voice synthesizer (16), a timer (13), a cassette interface (17), an RS232C interface (20), a switching matrix (21), and a line interface (22). Audio, pulse dial, and tone signals are routed among the components and the telephone line (51) by the switching matrix (21) and line interface (22). The line interface (22) provides a transformerless means for coupling signals to and from the telephone line (51) and also analyzes the status of, and signals present on, the telephone line (51).

TECHNICAL FIELD

The present invention relates to call processing in telephony and, inparticular, is an improved device for remotely activating, modifying,and deactivating automatic answer features and telephone company centraloffice call forwarding equipment. The present invention also relates toline interfacing apparatus in telephony and, in particular, is animproved device for transmitting and receiving signals over a standardtelephone line without using transformers.

BACKGROUND OF THE INVENTION

At present, a subscriber to a telephone service may use an automaticanswering machine, which may or may not be designed to record a messagefrom a calling party, or use an answering service. The subscriber mayalso use a call forwarding feature offered by the local telephonecompany which automatically routes incoming calls to a differenttelephone number.

Answering services and telephone company call forwarding features arenot available everywhere and, where available, require the periodicpayment of a fee for that service of feature. Furthermore, automaticanswering machines and telephone company call forwarding features mustbe activated and deactivated at the subscriber's telephone set andcannot be remotely activated or deactivated. If a subscriber has the useof a call forwarding feature and plans to be at two or more differentlocations during the day he must either return to his office betweenevery different location or resign himself to having some of his callsforwarded to a first location (to which the calls are routed) while heis at the second location.

Furthermore, he must return to his office to cancel the call forwardingfeature. The subscriber must also return to his office if he wishes tochange from automatic answer to call forward, or vice versa. Theselimitations are a major inconvenience to a busy subscriber whose time isvaluable and cannot be wasted on making numerous return trips to hisoffice. Under these conditions, the utility of conventional call forwardservice is limited.

Thus, there is a need for an improvement in telephone answering and callforwarding apparatus which will allow for remote activation,modification, and deactivation of an automatic answering device and thecall forwarding service provided by central office equipment.

Conventional devices which transmit or receive information overtelephone lines have one or more transformers to isolate the telephoneline from grounding by the device. Transformers often significantlyincrease the size, weight and cost of the devices in which they areused. Transformers also have a limited bandwidth, do not pass directcurrent, and therefore limit the amount of information that may beobtained from the telephone line. There is, therefore, a need for adevice which can transmit and receive information over telephone lineswithout transformers but which does not ground the telephone line.

SUMMARY OF THE INVENTION

The present invention fulfills the above need by providing a means forremotely activating, modifying, and deactivating automatic answeringfeatures and call forwarding features and for remotely changing thenumber to which incoming calls are to be forwarded. The presentinvention overcomes the drawbacks of the prior art by allowing thesubscriber to call his office from a different location and, by dialingin an access code and a predetermined command code, selectively activateand deactivate an automatic answering feature provided by the presentinvention and the call forwarding service provided by central officeequipment. The subscriber can also change the number to which his callsare to be forwarded, play back recorded messages by the priority of themessage, and change certain other parameters of operation, such as thenumber of incoming calls which will cause the present invention toautomatically cancel the call forwarding service. The subscriber mayalso selectively cause the present invention to cancel the callforwarding service either after a predetermined number of incoming callsor after a predetermined number of incoming calls within a predeterminedtime window.

A typical automatic answering device will answer an incoming call and,if desired, prompt the calling party to leave a message and then recordthe message left by the calling party. The present invention, inaddition to the above function, can also prompt the calling party tocall a different telephone number and allow the subscriber to remotelychange the telephone number to be called. The present invention can alsoprompt the calling party to dial in the telephone number where he can bereached and to dial a code indicating the urgency (priority) of hiscall. The subscriber can then remotely retrieve these messages, bypriority if desired. The subscriber will hear the telephone number andpriority, not as a series of tones, but as a series of spoken words andnumbers.

The present invention accomplishes these functions and improvements bymeasuring the D.C. voltages and currents present on the telephone line,by measuring the frequency content of signals on the telephone line, bymeasuring the duration of and counting the number of ringing signalspresent on the telephone line, by sinking current from the telephoneline, and by coupling tones and voice signals to the telephone line.

The present invention also uses both capacitance and high-resistancecoupling to transfer information to and from the telephone line, therebyavoiding the use of transformers for coupling. The present invention hasan isolated system ground and an isolated RS-232C interface to preventgrounding of the telephone line. This coupling method allows the presentinvention to measure the DC voltages and currents present on thetelephone line with simple, inexpensive circuits which do not disturbthe status of the telephone line.

Thus, it is an object of the present invention to provide apparatus forremotely activating, modifying, and deactivating automatic answerdevices and call forwarding service.

It is a further object of the present invention to provide a means fordetecting when a remote party has answered a call or has gone on-hook.

It is a further object of the present invention to provide improvedapparatus for quantifying signals which are present on a telephone line.

It is a further object of the present invention to provide a device foranswering incoming calls and for originating calls with a subscriberline interface which couples signals to or from the telephone linewithout using transformers or grounding the telephone line.

These and other objects of the present invention will become apparentupon reading the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the preferred embodiment in a typicaloperating environment.

FIG. 2 is a diagram of the telephone line interface circuit.

FIG. 3 is a schematic diagram of a rotary pulse and ring detector.

FIG. 4 is a schematic diagram of an isolation amplifier.

FIG. 5 is a schematic diagram of a DC feed and off-hook detector.

FIG. 6 is a diagram of a loop current sink/modulator and a hang-updetector.

FIGS. 7A and 7B are schematic diagrams of a tone detector.

DETAILED DESCRIPTION

Turning now to the drawing in which like numerals reference likecomponents throughout the several figures, the preferred embodiment ofthe present invention will be described. FIG. 1 is a block diagram ofthe preferred embodiment 10 of the present invention in a typicaloperation environment.

The address output of control unit 11 and the address inputs of memory12 and timer 13 are connected via eight-bit address bus 23. The datainputs and outputs of control unit 11, memory 12, timer 13 and externalinterface 20, the data inputs of tone encoder 15 and voice synthesizer16, and the data outputs of tone decoder 14 and line interface 22, areall interconnected via eight-bit data bus 25. The control signal inputsand outputs of control unit 11, timer 13, cassette interface 17,external interface 20, and line interface 22, and the control inputs ofmemory 12, tone decoder 14, tone encoder 15, voice synthesizer 16, andswitching matrix 21 are interconnected by one or more of the pluralityof conductors comprising control bus 24.

A first set of the signal inputs and outputs of switching matrix 21 andline interface 22 is connected via signal bus 35, which is comprised offour conductors. A second set of the signal inputs of switching matrix21 is connected to the outputs of tone encoder 15, voice synthesizer 16and cassette interface 17, by conductors 27, 30 and 32, respectively. Asecond set of the signal outputs from switching matrix 21 is connectedto the input of tone decoder 14 via conductor 26, and to the inputs ofcassette interface 17 via conductors 33 and 34. Finally, a conductor 31connects a bidirectional input/output port of switching matrix 21 to abidirectional input/output port of cassette interface 17.

Line interface 22 is connected to the telephone company central office50 via at least one subscriber line 51, and to a termination device 43via extension line 47, where lines 47 and 51 each have at least twoconductors.

Cassette interface 17 is connected to a cassette tape recorder/playbackunit 40 via bus 44, and to a cassette playback unit 41 via bus 45. Buses44 and 45 are comprised of the plurality of conductors necessary tocontrol the operation of the particular cassette units 40 and 41.

It will be appreciated by one skilled in the art that, except for theinterrupt signal from timer 13 on conductor 28, control unit 11 reads ormonitors the outputs of memory 12, timer 13, tone decoder 14, and lineinterface 22 by placing the appropriate signals on address bus 23 and/orcontrol bus 24, thereby causing the selected component 12, 13, 14 or 22to place its outputs on data bus 25, and then allowing the inputs ofcontrol unit 11 to become responsive to signals on data bus 25. It willalso be appreciated by one skilled in the art that control unit 11causes or commands a selected component 12, 13, 14 or 22 to place itsoutput on data bus 25 in the same manner, and commands a selectedcomponent 12, 13, tone decoder 15, voice synthesizer 16, switchingmatrix 21, or line interface 22 to be responsive to signals on data bus25 in the same manner. It will therefore be understood that control unit11 commands components 13-17, 21 and 22 to read data words from or writedata words into memory 12 by placing appropriate signals on address bus23 and control bus 24. It will also be understood that control unit 11may place a data word on data bus 25 and command a selected component tobe responsive to that data word.

The term "digit" as used herein refers to the numbers 0 through 9 andthe characters * and # on a conventional 3×4 telephone keypad. It willbe appreciated that to dial a digit means to place on a conductor thedual tone multiple frequency (DTMF) electrical signal corresponding tothat digit.

Memory 12 is comprised of a read only memory (ROM), which contains theoperating instructions, and a random access memory (RAM), which is usedfor the temporary storage of data. The signals placed on address bus 23and control bus 24 by control unit 11 determine whether a data word isto be read from the ROM or whether a data word is to be read from orwritten into the RAM.

Timer 13 is a quadruple timer unit, such as the Z80A CTC, currentlymanufactured by Mostek Corporation as part number MK3882. Timer 13, inresponse to commands from control unit 11, reads timing parameters fromdata bus 25 and places data indicating the status of its timers on databus 25. Timer 13 receives timer triggering signals from cassetteinterface 17 and line interface 22 via control line 24, sends time-outsignals to cassette interface 17 via control bus 24, and sends aninterrupt signal to control unit 11 via conductor 28. Each timer intimer 13 can be independently commanded by control unit 11 to begin atiming period in response to a timer triggering signal and to place atime-out signal on conductor 28 at the end of the period. Each timer canalso be independently commanded by control unit 11 to increment ordecrement its contents in response to a timer triggering signal andplace a time-out signal on control bus 24 when its contents are zero oroverflow.

Tone decoder 14 receives DTMF electrical signals on conductor 26 and,when commanded by control unit 11, places on data bus 25 a digital wordwhich corresponds to the digit represented by the DTMF electricalsignal. It will be appreciated that control unit 11 may command tonedecoder 14 to place on data bus 25 a series of digital words whichrepresent the series of digits corresponding to a series of DTMFelectrical signals present on conductor 26. Tone decoder 14 is connectedto switching matrix 21 via conductor 26. As will be discussed in moredetail below, control unit 11 may command line interface 22 andswitching matrix 21 to place on conductor 26 the electrical signalspresent on either subscriber line 51 or extension line 47.

Tone encoder 15, when commanded by control unit 11, will read thedigital word present on data bus 25, generate a DTMF electrical signalcorresponding to the digit represented by the digital word, and placethe DTMF electrical signal onto conductor 27. It will be appreciatedthat control unit 11 may command tone encoder 15 to read a series ofdigital words present on data bus 25 and place a series of DTMFelectrical signals onto conductor 27. Tone encoder 15 is connected toswitching matrix 21 via conductor 27. As will be discussed in moredetail below, control unit 11 may command switching matrix 21 and lineinterface 22 to place this DTMF electrical signal onto subscriber line51 or extension line 47.

Voice synthesizer 16, when commanded by control unit 11, will read thedigital word present on data bus 25, generate an audio electricalsignal, and place this signal onto conductor 30. It will be appreciatedthat in response to an appropriate series of digital words synthesizer16 will output the sound of a spoken word. In the preferred embodimentof the present invention, the ROM of memory 12 contains a plurality ofsets of digital words. Control unit 11 may, therefore, command voicesynthesizer 16 to read these words and to place onto conductor 30 aseries of electrical signals which correspond to the sound of a voicespeaking numbers 0 through 9 and a predetermined number of other vocalwords or sounds.

Voice synthesizer 16 is comprised of a conventional digital-to-analogconverter (DAC) and other support components. It will be appreciatedthat using a DAC provides a high-quality human voice sound, but alsorequires a substantial amount of memory 12 space, thus increasing thetotal cost of the device 10. In some applications, part or all of theuser's voice requirements may be met by a lesser-quality voice, i.e.,one which sounds computer-generated. In such applications, voicesynthesizer 16 may be comprised of a DAC and/or a phoneme speechsynthesizer, plus other support components. One example of a phonemespeech synthesizer is the SSI 263A Phoneme Speech Synthesizer, currentlymanufactured by Silicon Systems of Tustin, Calif. Using a phoneme speechsynthesizer for part of all of the user's requirements will save memory12 space and lower the cost of the device.

Cassette interface 17 is connected to cassette tape recorder/player unit40 and cassette tape player unit 41 via tape buses 44 and 45,respectively, to switching matrix 21 via conductors 31, 32, 33 and 34,and to control unit 11 via control bus 24. Cassette interface 17 buffersaudio and control signals between cassette units 40 and 41 and switchingmatrix 21. Cassette interface 17 also buffers control signals betweencontrol bus 24 and cassette units 40 and 41. The design of a cassetteinterface unit 17 will depend upon the characteristics of the particularcassette units 40 and 41 used by the subscriber. It will be understoodfrom the later detailed description of line interface 22 that cassetteinterface 17 must prevent cassette units 40 and 41 from connecting thesystem ground of device 10 to an external ground since the system groundis directly connected to the subscriber line 51. Means for accomplishingthe above are well known to those skilled in the art. Cassette units 40and 41 may be embodied by a single cassette unit.

Switching matrix 21, in the preferred embodiment, is constructed fromtwo interconnected 4×4 crosspoint switches such as the Motorola 1452100or RCA CD22100 devices. Each crosspoint switch has eight bidirectionalports, and one of the ports of the first crosspoint switch is connectedto one of the ports of the second crosspoint switch such that the resultis equivalent to a 4×7 crosspoint switch. Switching matrix 21selectively routes audio signals among tone decoder 14, tone encoder 15,voice synthesizer 16, cassette interface 17, and line interface 22 inresponse to commands from control unit 11. The method by which switchingmatrix 21 accomplishes routing of audio signals in response to commandsfrom control unit 11 is known to those skilled in the art.

Line interface 22 monitors the electrical signals present on subscriberline 51 and extension line 47 and, in response to commands from controlunit 11, places digital status words onto data bus 25. There are twostatus words. Each status word has a plurality of bits, and each bitrepresents a specific condition such as: the termination device 43 inon-hook or off-hook, central office 50 is placing a ringing signal onsubscriber line 51, termination device 43 is dialing a number by rotarydialing, central office 50 has hung up subscriber line 51, that thecharacteristics of the electrical signals present on subscriber line 51or extension line 47 are indicative of a human voice, or a sound otherthan a human voice, and that certain tones are present.

Line interface 22 receives and buffers electrical signals present onsubscriber line 51 and places them on signal bus 35 which is connectedto switching matrix 21. Line interface 22 also receives and bufferselectrical signals placed on signal bus 35 by switching matrix 21 andthen places these electrical signals onto on subscriber lines 51. Lineinterface 22, also on command by control unit 11, receives and bufferssignals from, and places signals onto subscriber line 51 and extensionline 47. Unless control unit 11 command the contrary, line interface 22maintains a connection between subscriber line 51 and extension line 47so that termination device 43 will receive and originate telephone callsin the normal manner. Line interface 22 also provides operating power totermination device 43 when control unit 11 commands line interface 22 tobreak the connection. The details of line interface 22 will be describedlater.

In view of the above description of switching matrix 21 and lineinterface 22 it will be understood that to "connect" a component toanother component, e.g.--termination device 43 to tone decoder 14, andto "route" a signal, e.g.--the output signal of termination device 43 tothe input terminal of tone decoder 14, is an unambiguous manner ofspecifying the operation of line interface 22 and/or switching matrix 21to obtain a desired result.

External interface 20 is an RS-232 interface to allow the preferredembodiment to communicate with an external device 42 over an externalbus 47. The design of interface 20 and the number of conductors in bus46 will necessarily depend upon the requirements of the external device42. External interface 20 is connected to control unit 11 via controlbus 24 and data bus 25.

As will be apparent from the later discussion of line interface 22,subscriber line 51 is connected to the system ground of the presentinvention 10. The system ground is therefore necessarily floating withrespect to the ground of the local A.C.-power system so as not tounbalance subscriber line 51 and cause a line trouble signal to begenerated at central office 50. It will therefore be appreciated thatexternal interface 20 must isolate external bus 46 from the systemground. This is preferably accomplished by means of optocouplers inexternal interface 20 to isolate bus 46 from buses 24 and 25.

Assume now that the user of the preferred embodiment desires to set thedevice 10 in a first state so that, in response to an incoming call, itanswers the call and plays a message from cassette player 41. Thismessage prompts the calling party to leave a message which is recordedby tape recorder 40. In order to achieve this result, the user willcause termination device 43 to go off-hook, dial a predeterminedsequence of digits, and hang up. Termination device 43 is normally atelephone set. Line interface 22 will detect that termination device 43has gone off-hook. Since the outputs of line interface 22 are monitoredby control unit 11, when termination device 43 goes off-hook, controlunit 11 will command line interface 22 and switching matrix 21 toconnect tone decoder 14 to termination device 43. Tone decoder 14 willthen convert the sequence of digits dialed into a series of digitalwords. Control unit 11 reads the outputs of tone decoder 14 and, if thefirst digital word output by tone decoder 14 corresponds to a first or asecond predetermined digit, e.g., a * or #, then control unit 11 willcommand line interface 22 to disconnect tone decoder 14 and terminationdevice 43 from subscriber line 51, thereby disconnecting central office50. Control unit 11 then compares the remainder of the digital wordsfrom tone decoder 14 with predetermined code words stored in memory 12.

The preferred embodiment may also be controlled from a remote locationat which a telephone set embodies termination device 54. The user firstdials the phone number for subscriber line 51. Central office 50 willplace a ringing signal on subscriber line 51. Line interface 22 willdetect this ringing signal. Control unit 11 will then command lineinterface 22 to seize (answer) subscriber line 51 and also command lineinterface 22 and switching matrix 21 to connect tone decoder 14 andvoice synthesizer 16, to subscriber line 51. Control unit 11 thencommands memory 12 to place a series of digital words onto data bus 25,and commands voice synthesizer 16 to read this series of digital words.This series of digital words causes voice synthesizer 16 to produce aseries of tones for a first predetermined amount of time. To the user,this series of tones means that device 10 is responsive to commands fromthe user. To another calling party, this series of tones merely meansthat the call has been answered. During the tone sequence, the user thendials a predetermined sequence of digits. Tone decoder 14 will thenconvert the sequence of digits dialed into a series of digital words.Control unit 11 reads the outputs of tone decoder 14. If the first digitdialed is the first or the second predetermined digit, e.g., a * or a #,then control unit 11 will compare the remainder of the digital wordsfrom tone decoder 14 with predetermined code words stored in memory 12.

In either case, i.e., regardless of whether the user was usingtermination device 43 or 54, if the remainder of the digital wordsmatches the proper predetermined code word then, after the activetermination device 43 or 54, goes on-hook, control unit 11 places thepreferred embodiment in its first defined state and commands lineinterface 22 and switching matrix 21 to disconnect tone decoder 14 andvoice synthesizer 16 from the termination device 43.

Assume now that a calling party other than the owner of the preferredembodiment dials the user's telephone number from another telephone setshown as termination device 54. Control unit 11 causes the incoming callto be answered in the manner described above and then commands voicesynthesizer 16 to play the tone sequence. The calling party, not beingthe user, will normally not dial the first or the second predetermineddigit within a first predetermined period of time after subscriber line51 has been seized. Control unit 11 therefore commands switching matrix21 to disconnect voice synthesizer 16 and tone decoder 14, and toconnect cassette interface 17 to subscriber line 51, and then commandscassette interface 17 to cause cassette tape player 41 to play aprerecorded message to the calling party which prompts the calling partyto leave a message.

After the message has been played control unit 11 commands cassetteinterface 17 to cause tape player 41 to rewind and to cause taperecorder 40 to begin recording the calling party's reponse. A secondpredetermined period of time after the record command, or as soon as thecalling party hangs up, control unit 11 commands cassette interface 17to cause tape player 40 to stop recording. Control unit 11 then commandsline interface 22 and switching matrix 21 to disconnect cassetteinterface 17, and subscriber line 51 from each other, and commands lineinterface 22 to hang up subscriber line 51.

Assume now that the user desires to listen to the responses made by thecalling party. The user will cause termination device 43 to go off-hookand dial a different predetermined sequence of digits. Line interface 22will detect the off-hook condition of termination device 43 and controlunit 11 will command line interface 22 and switching matrix 21 toconnect tone decoder 14 to termination device 43. Tone decoder 14converts this predetermined sequence of digits into a series of digitalwords which are read by control unit 11. If the first digit dialed isthe first or second predetermined digit described above, then controlunit 11 will command line interface 22 to disconnect tone decoder 14 andtermination device 43 from subscriber line 51. Control unit 11 will thencontinue monitoring the output of tone decoder 14.

Alternatively, if at a remote location, the user can cause terminationdevice 54 to go off-hook and dial the user's telephone number. In thiscase line interface 22 will detect the predetermined ringing signal.Control unit 11 will command line interface 22 to answer subscriber line51, command line interface 22 and switching matrix 21 to connect tonedecoder 14 and voice synthesizer 16 to subscriber line 51, and commandvoice synthesizer 16 to play the tone sequence for the firstpredetermined amout of time. During this time the user will dial thepredetermined sequence of digits. Tone decoder 14 converts thepredetermined sequence of digits into a series of digital words whichare read by control unit 11. If, within the first predetermined amountof time after the answer of subscriber line 51, the first digit dialedwas the first or second predetermined digit described above, controlunit 11 will continue monitoring the output of tone decoder 14 and willcommand switching matrix 21 to disconnect voice synthesizer 16 fromsubscriber line 51.

In either case, control unit 11 then compares the remainder of thedigital words from tone decoder 14 with predetermined code words inmemory 12. If the remainder matches the appropriate code word thencontrol unit 11 will command switching matrix 21 to connect cassetteinterface 17 to subscriber line 51, or extension line 47, asappropriate. Depending upon the particular code word dialed, controlunit 11 will command cassette interface 17 to cause tape recorder 40 toplay back the recorded responses, cause tape player 41 to record a newmessage, or cause tape recorder 40 and/or tape player 41 to pause, stop,rewind, fast forward, or erase.

The remainder of the digital words may also correspond to a differentcode word in memory 12 which instructs control unit 11 to place thepresent invention 10 in one of the different states described below, orto simply cancel operation in the first state. As previously stated,control unit 11 continuously monitors the outputs of line interface 22so that when the user causes termination device 43 to go on-hook,control unit 11 will, as previously described, command switching matrix21 and line interface 22 to disconnect any connected components and toreconnect termination device 43 to subscriber line 51. If the user hascalled from termination device 54 control unit 11 will also command lineinterface 22 to hang up subscriber line 51.

Assume now that the user desires to set the device 10 in a secondoperating state so that an incoming call is answered and a prerecordedmessage from tape player 41 is provided to the calling party. Thismessage prompts the calling party to dial his telephone number and adigit indicating the perceived urgency of his call. The device 10 thenstores, in memory 12, the telephone number and urgency code of thecalling party. The user places device 10 in a state to respond toincoming calls in this manner by dialing the * or # digit, followed byanother predetermined sequence of digits. The operation of device 10 isas described above until the predetermined code is detected.

Assume now that the user has decided to find out if any calling partyhas left a telephone number for the user to call. In the same manner aspreviously described, the user causes termination device 43 to gooff-hook and dial the appropriate predetermined sequence of digits.Alternatively, the user may cause a remote termination device 54 to gooff-hook and then dial the user's telephone number. Then, within thefirst predetermined amount of time, after the device 10 has answeredsubscriber line 51, the user dials the appropriate predeterminedsequence of digits. This sequence of digits instructs control unit 11 torecall the first message stored in memory 12.

As previously described, voice synthesizer 16 provides an audio outputsignal in response to a series of digital words. Therefore, the userwill hear spoken numbers which corresponds to the digits of the callingparty's telephone number, and the urgency code of the call. Control unit11 will then await further instructions from the user. If the user thendials another predetermined sequence of digits control unit 11 will, inthe manner described above, cause the user to be provided with thetelephone number and urgency code of the second calling party, and so onuntil all of the calling party telephone numbers and urgency codes havebeen provided to the user. Control unit 11 will also cause apredetermined tone burst to be placed on subscriber line 51 or totermination device 43, as appropriate, thus alerting the user that thereare no more recorded messages. Control unit 11 will then await furtherinstructions from the user in the form of other predetermined sequencesof digits, to replay the messages, erase the messages, or change device10 to a different state of operation.

The user may also instruct the device 10 to play back the highestpriority messages first. If, after dialing the appropriate sequence ofdigits corresponding to the predetermined code word, the user dialsanother predetermined sequence of digits control unit 11 will cause theuser to be provided, in the same manner described above, the telephonenumber and urgency code of the calling party with those having thehighest urgency level being provided first.

Assume now that the user desires to set the device 10 in a third stateof operation so that the device 10 answers an incoming call and plays aprerecorded message to the calling party. This message advises thecalling party of a different number where the user can be reached. Theuser sets the device 10 in this third mode of operation in the samemanner previously described, i.e.--causing termination device 43 to gooff-hook and dial a predetermined sequence of digits, or causing aremote termination device 54 to go off-hook, dial the user's telephonenumber and, after answering, dial a predetermined sequence of digits.The user then causes the termination device 43 or 54 to dial anotherpredetermined sequence of digits followed by a sequence of digitscorresponding to the telephone number at which he can be reached.Control unit 11 stores the sequence of digits corresponding to thetelephone number as digital words in memory 12.

Assume now that a calling party dials the user's telephone number.Control unit 11 will command line interface 22 and switching matrix 21to connect voice synthesizer 16 to subscriber line 51, and command voicesynthesizer 16 to play the tone sequence. If the calling party does notdial the first or second predetermined digit within the firstpredetermined period of time then control unit 11 will command memory 12to send a predetermined series of digital words to voice synthesizer 16,and also command voice synthesizer 16 to read this series of digitalwords from memory 12 and to provide a corresponding audio output signalto subscriber line 51. This audio output signal will be speech whichadvises the calling party that the user is not currently at the numberdialed, but can be reached at a different number. Control unit 11 thenreads from memory 12 the sequence of digital words corresponding to thesequence of digits of the different telephone number and provides theseto voice synthesizer 16. The calling party will then hear a spokennumber corresponding to each of the digits of the telephone number wherethe user can be reached.

Assume now that the user is serviced by a telephone company centraloffice 50 which provides a call forwarding feature. In order to activatethe call forwarding feature, central office 50 typically requires thatthe user place onto subscriber line 51 a predetermined sequence ofdigits, typically the digits 73#, followed by a sequence of digitscorresponding to the telephone number to which the user desires thecalls be forwarded (the destination telephone number), hang up and, ifthe destination number was busy or did not answer, repeat this sequence.In order to deactivate the call forwarding feature, central office 50typically requires that the user place onto subscriber line 51 apredetermined sequence of digits, normally the digits 72#, and hang up.In the United States, most offices embodying central office 50 place aburst of the ringing signal on subscriber line 51 in response to eachincoming call when the call forwarding feature is active. This burst ofthe ringing signal is a courtesy "reminder" ring and is typicallyone-quarter to one-half of a normal ring burst.

The present invention provides apparatus for activating and deactivatingcentral office call forwarding equipment from a remote location. This inturn provides apparatus by which the user may change the destinationnumber without returning to the site at which the preferred embodimentis located.

In the foregoing description of the operation's central office callforwarding equipment, it would be apparent that these sequence of stepswhich command the central office to initiate and cancel a callforwarding feature must be executed from a termination device connectedto the user's subscriber line 51. Once forwarding is initiated, it canonly be terminated by execution of the steps defined by the operatingcompany operating central office 50, executed again from a terminationdevice connected to the subscriber's telephone line 51.

The approach of the preferred embodiment of the present invention is tohave device 10 detect a predetermined condition which will automaticallycause the apparatus to seize subscriber line 51 and place the sequenceof DTMF digits on the line which deactivates the forwarding feature.Once this has been accomplished, incoming calls from remote locations tosubscriber line 51 can be answered in a normal fashion. This allows theuser to then reactivate the call forwarding feature to a differentdestination number.

Naturally, all of the steps described below for operating the callforwarding control features of the present invention may be executedfrom the local termination device 43. Control unit 11 monitors thestatus and output of termination device 43. The user may therefore usetermination device 43 to directly activate the call forwarding feature.In this case control unit 11 will recognize that the call forwardingfeature has been activated and will respond to the reminder rings asdescribed below. Alternatively, the user may use termination device 43to instruct control unit 11 to perform the steps necessary to activatethe call forwarding feature and to respond to the reminder rings asdescribed below. However, the primary utility of this aspect of thepresent invention is the remote activation and cancellation of the callforwarding feature.

As in its other states of operation, the preferred embodiment employstone decoder 14, tone encoder 15, switching matrix 21, counter timer 13and line interface 22 to implement the process described herein.

Upon receipt of an incoming call, control unit 11 causes the preferredembodiment to provide the predetermined time window for the user to dialdigits indicating that a predetermined sequence of digits constituting acommand to the preferred embodiment will follow. Assume for the momentthat call forwarding is inactive and the user is at a remote numberwhich he wishes to define as the destination number when the callforwarding feature is activated. The predetermined sequence indicatingthat the digits to follow are the destination number and that callforwarding is to be activated are provided by the user. This informationis stored and, upon termination of the call, control unit 11 causes theapparatus to execute the following steps. Line interface 22 goesoff-hook and the predetermined code recognized by the central office(73#) is provided over the line, followed by the digits corresponding tothe destination number. The apparatus then hangs up, reseizes the line,and again dials the digits corresponding to the destination number. Inother words, after receiving the command to initiate forwarding to apredetermined destination number, the device will seize subscriber line51 and execute the same sequence of steps that a subscriber using anormal telephone on a line with the call forwarding feature wouldexecute.

In the absence of the preferred embodiment, once the call forwardingfeature is activated it can only be deactivated by operation of thedevice connected to subscriber line 51. However, the present inventionprovides apparatus which will automatically cancel the call forwardingfeature in response to detection of a predetermined condition.

In the preferred embodiment, the predetermined condition is a receipt ofN calls within a predetermined time period P. After control unit 11 hassuccessfully caused the apparatus to initiate the call forwardingfeature, it is then operating the apparatus in a state corresponding tothe call forwarding feature being active. Therefore, the unit is thestate in which it is waiting to detect occurrence of the predetermineddeforwarding condition. Line interface 22 is responsive to detection ofring signals on subscriber line 51. Therefore, control unit 11 willcount and store the number of the reminder ring bursts which occur onthe line during a predetermined time defined by counter timer 13. In thepreferred embodiment, this is a moving time window. Therefore countertimer 13 may either concurrently keep track of several timing periods,or information concerning the elapsed time from receipt of the firstreminder ring burst after the call forwarding active state has beenentered may also be stored in memory. Each time another reminder ringburst is detected, the control unit 11 will check all stored times ofoccurrence of the reminder ring burst to see if the condition of apredetermined number N of the bursts occurring within time period P hasoccurred.

The preferred embodiment provides default values for N and P but thesemay also be set by the user.

Naturally, this arrangement is one for which unintentional deforwardingcan occur if the user's telephone number receives an abnormally largenumber of calls within a short period of time. However, in usingpractical embodiments of the present invention, the user will befamiliar enough with his or her normal call receipt rate so that N and Pmay be selected to define a number of calls per unit time which is (a)and likely to occur in response to the normal incoming call traffic tothe user's line; and (b) does not represent such a high density of callsthat the appropriate number cannot be made within time period P by theuser in order to force deactivation remotely. For example, if N ischosen to be 3 and P is chosen as a ninety second period, this mayrepresent an unlikely occurrence in response to the user's normal callsreceived from third parties. However, when the user is at a remotelocation corresponding to the destination telephone number, and wishesto change the destination number to a new number, the user will executethe following steps. First, the user's number is dialed. If the callforwarding feature has already been deactivated because of an unusuallylarge number of third parties dialing the user's telephone number, theuser will receive the above described tone bursts indicating thatincoming calls are being answered by the preferred embodiment. When thisoccurs, the user can either play back any messages which may have beenleft, or command the preferred embodiment to forward calls to the newdestination number.

Assuming the more likely occurrence, that forwarding is still in effect,the user simply dials his or her subscriber telephone number up to Ntimes within the predetermined period and deforwarding is accomplished.When the number of forwarded third party calls plus the number of timesthe subscriber has dialed his or her own telephone number equals N, thenthe present invention will, as described below, deactivate the centraloffice call forwarding feature. The next time the user dials a numberthe call will then be answered by the preferred embodiment. The userwill then dial the predetermined sequence of digits commanding thepreferred embodiment to accept the following sequence of digits as a newdestination number and to activate the central office call forwarding.

In response to receipt of N reminder ring bursts within time period Pthe preferred embodiment seizes line 51 and provides to the centraloffice the predetermined sequence of digits (normally 72#) to deactivatethe call forwarding. Control unit 11 then forces the preferredembodiment into its state in which it is awaiting to receive incomingcalls. Naturally, if the user wishes to place the apparatus in anotherstate of answering calls, he or she may do so as described hereinabove.

Assume now that the user desires to set the device 10 in a state ofoperation so that the device 10 will originate calls to predeterminedtelephone numbers, wait for an answer, play a prerecorded message, andthen hang up. The user will, as described previously, cause terminationdevice 43 or 54, as appropriate, to now dial a predetermined sequence ofdigits and hang up. Control unit 11 will detect the off-hook conditionor ringing signal, detect the digits dialed, all in the mannerpreviously described, and place the device 10 into the sixth state ofoperation.

Control unit 11 will then command line interface 22 to seize subscriberline 51, command line interface 22 and switching matrix 21 to connecttone encoder 15 to subscriber line 51. Memory 12 and tone encoder 15then place onto subscriber line 51 a sequence of digits corresponding tothe first predetermined telephone number to be called. Control unit 11then commands line interface 22 and switching matrix 21 to disconnecttone encoder 15 from subscriber line 51.

Control unit 11 then monitors the output of line interface 22 for thepresence of a human voice, which indicates that the called party hasanswered. If the called party has not answered within a predeterminedamount of time, control unit 11 commands line interface 22 to hang upsubscriber line 51. This sequence is then repeated for the secondpredetermined telephone number to be called, then the third, and so on.However, if the called party answers, line interface 22 will detect thepresence of speech on the line. Control unit 11 will then commandswitching matrix 21 to disconnect tone encoder 15 from subscriber line51. Control unit 11 will then, in accordance with the user's priorinstruction command switching matrix 21 to connect subscriber line 51 toa human operator at termination device 43, or connect cassette interface17 to subscriber line 51. If the latter, control unit 11 will thencommand cassette interface 17 to cause cassette tape player 41 to placea prerecorded message on subscriber line 51. When the prerecordedmessage is finished, when termination device 43 goes on-hook, or whenthe called party hangs up, whichever occurs first, control unit 11commands, as appropriate, cassette interface 17 to cause tape player 41to rewind, commands switching matrix 21 to disconnect cassette interface17, and commands line interface 22 to hang up subscriber line 51.Control unit 11 then advances to the next predetermined telephonenumber.

Assume now that the user has not instructed the device 10 to perform anyfunctions; the device 10 will therefore be in its default operationstate. If the user desires to place the device 10 in any of itsoperational states, the user may do so by using termination device 43 inthe manner previously described. The user may also place the device 10in any of its operational states by using a remote termination device54. The user will dial the user's telephone number. If the call is notbeen answered by a person using termination device 43 after apredetermined number of ringing signals, control unit 11 will commandline interface 22 to answer the call by seizing subscriber line 51.Control unit 11 then places a brief answer acknowledgment tone onsubscriber line 51. Control unit 11 then monitors the output of tonedecoder 14. The user can then place the device 10 in any of itsoperational states in the manner previously described. Control unit 11will cause line interface 22 to hang up subscriber line 51 if a validcommand is not received within the predetermined amount of time or ifcentral office 50 disconnects subscriber line 51.

It will be appreciated that the user can prevent unauthorized tamperingwith the device 10 by requiring that the calling party provide, by DTMFdialing, a security code before control unit 11 will be responsive toany commands. It will also be appreciated that the device 10 also hasother advanced telephone features such as last number redial and speeddialing from a bank of telephone numbers stored in memory 12.

The foregoing has been a description of the operation of the preferredembodiment in connection with the block diagram of FIG. 1. The followingdescriptions of the appartus shown on the other drawing figures showcircuit details of the preferred embodiment.

Turn now to FIG. 2 which is a diagram of the line interface 22. Thearrows superimposed on the conductors are indicative of the direction ofsignal flow on the conductor. It will be appreciated that someconductors have bidirectional signal flow.

Line interface 22 is a transformerless line interface and comprises avoltage suppressor 106, a noise suppressor capacitor 107, two systemground reference diodes 103 and 104, a rotary/ring detector 110, twoisolation amplifiers 113, a speech detector 115, a loop currentsink/modulator 120, a hangup detector 124, a line connection circuit130, a muting circuit 139, a DC feed/off-hook detector 155, an inputlatch 108, two output latches 109, a full-wave bridge rectifier 125 anda tone detector 161. Line interface 22 is connected to subscriber line51, extension line 47, control bus 24, data bus 25, and signal bus 35.

Subscriber line 51 comprises two conductors, commonly referred to as thecentral office tip 101 and the central office ring 102. A surge voltagesuppressor 106 is connected between conductors 101 and 102 to limit thevoltage between these conductors to a predetermined safe value.Capacitor 107 is connected between conductors 101 and 102 to suppresshigh frequency noise. System ground references diodes 103 and 104 areconnected, in parallel, but with opposite electrical orientation,between conductor 102 and, via conductor 105, system ground. Diodes 103and 104 therefore allow electrical signals on conductor 101 to bereferenced to system ground.

The data inputs of latch 108 are connected to data bus 25, and thegating input is connected to control bus 24 via conductor 118. The dataoutputs of latch 108 are connected to the inputs of loop currentsink/modulator 120, line connector 129, DC feed and off-hook detector155, muting circuit 139, and the two inputs of tone detector 161 byconductors 121, 131, 154, 140, and 158 and 159, respectively. It will beappreciated that control unit 11 controls the operation of components120, 129, 155, 139, and 161 by first placing data on data bus 25, andthen placing a gating signal on conductor 118 of control bus 24. Thiscauses latch 108 to read and store the data present on data bus 25 andplace this data on its output.

The data inputs of tri-state latch 109-1 are connected to the outputs ofrotary/ring detector 110 by conductors 111 and 112, to the outputs ofspeech detector 115 by conductors 116 and 117, to the output of hangupdetector 124 by conductor 126, and to the output of DC feed and off-hookdetector 155 by conductor 153. The data inputs of tri-state latch 109-2are connected to the outputs of tone detector 161 by conductors 162,163, 164 and 165. The control inputs of latches 109-1 and 109-2 areconnected to control bus 24 by conductors 119-1 and 119-2, respectively.The outputs of latches 109-1 and 109-2 are connected to data bus 25. Itwill therefore be appreciated that control unit 11 reads the outputs ofcomponents 110, 115, 124, 155 and 161 by placing a control signal onconductor 119-1 or 119-2, as appropriate, thereby causing latch 109-1 or109-2 to place its outputs on data bus 25. Control unit 11 then readsthe data present on data bus 25.

Conductor 101 of subscriber line 51 is connected to one of the AC inputsof the full-wave bridge rectifier 125, and conductor 102 is connected tothe other AC input of bridge rectifier 125. The positive output terminalof bridge rectifier 125 is connected to conductor 128 and the negativeoutput terminal is connected to conductor 138. Bridge rectifier 125therefore changes the battery, reverse battery, and ringing signalsplaced on subscriber line 51 by central office 50 into a unipolar DCsignal for the operation of loop current sink and modulator 120, DC feedand off-hook detector 155, and muting circuit 139.

Line connector 129 comprises an insulated gate field effect transistor132, a diode 134, and a relay 130. The gate of transistor 132 isconnected to one of the outputs of latch 108 by conductor 131, thesubstrate and source are connected to system ground by conductor 136,and the drain is connected by conductor 133 to one end of the parallelcircuit of diode 134 and the coil of relay 130. The other end of thisparallel circuit is connected by conductor 135 to the system logic powersupply VL. The cathode of diode 134 is connected to conductor 135 andthe anode is connected to conductor 133 so that diode 134 suppressesvoltage and current surges from the coil of relay 130. Relay 130 is asingle-pole, single-throw normally closed relay. One contact point ofrelay 130 is connected to conductor 128 and the other contact point isconnected to conductor 127. Extension line 47 comprises conductor 127and 138. It will therefore be appreciated that line connector 139connects and disconnects subscriber line 51 and extension line 47 byconnecting and disconnecting conductors 128 and 127.

It will be understood that when the appropriate output of latch 108places on conductor 131 a positive voltage greater than the thresholdvoltage of transistor 132, transistor 132 will turn on, thereby allowingcurrent to flow from the system logic power supply through the coil ofrelay 130 and transistor 132 to system ground, thereby causing relay 130to open its contacts and disconnect conductor 128 from conductor 127.

Muting circuit 139 comprises transistor 142 and resistors 141 and 146.Conductor 127 is connected to conductor 138 by the series combination ofresistor 146, conductor 145, and the collector and emitter of transistor142. The base of transistor 142 is connected to one of the outputs oflatch 108 by the series combination of conductor 147, resistor 141, andconductor 140. It will be understood that when the appropriate output oflatch 108 places on conductor 140 a voltage greater than the thresholdvoltage of transistor 142, transistor 142 will be turned on. Thiseffectively shorts conductors 127 and 138 together and mutes allelectrical signals on this pair of conductors.

FIG. 3 is a schematic diagram of rotary/ring detector 110. Conductor 101of subscriber line 51 is connected to one terminal of a DC blockingcapacitor 170. The other terminal of capacitor 170 is connected byconductor 171 to one of the AC inputs of the full-wave bridge rectifier172. Conductor 102 is connected to the other AC input of bridgerectifier 172.

It will be appreciated that the purpose of capacitor 170 is to block theDC battery voltage provided by central office 50 and to pass AC signalssuch as rotary dial pulses and ringing signals. The positive outputterminal of bridge rectifier 172 is connected to the negative outputterminal of rectifier 172 through the series combination of conductor173, resistor 174, conductor 175, the parallel circuit of resistor 177and the diode 178-a of optocoupler 178, and conductor 176. The cathodeof the diode 178-a of optocoupler 178 is connected to conductor 176 andthe anode is connected to conductor 175.

It will be appreciated by one skilled in the art that optocoupler 178typically comprises a light emitting diode 178-a and a transistor 178-band that the transistor 178-b becomes conductive whenever a currentgreater than a predetermined threshold current flows through the diode178-a. It will therefore be appreciated that the values of resistors 174and 177 are not critical but resistor 174 must be sufficiently large andresistor 177 must be sufficiently small to prevent loading of subscriberline 51 and excessive current flow through the diode 178-a. Also,resistor 174 must be sufficiently small and resistor 177 must besufficiently large to permit sufficient current to flow through thediode 178-a to reliably cause transistor 178-b to become conductive.

The emitter of transistor 178-b is connected to the system ground byconductor 179. The collector of transistor 178 is connected to one endof resistor 181 by conductor 180. The other end of resistor 181 isconnected by conductor 184 to one end of resistor 182, to one end ofcapacitor 185, and to the input of the schmitt-trigger inverter 187. Theother end of resistor 182 is connected to the system logic voltagesupply VL by conductor 183. The other end of capacitor 185 is connectedto the system ground by conductor 186. The output of inverter 187 isconnected by conductor 111 to the positive-going input trigger ofmonostable 188 and also to one of the data inputs of latch 109-1.

It will be appreciated that components 181, 182 and 185 form a low passfilter with its output on conductor 184. This filter passes rotary pulseand ringing information but attenuates higher frequency audio and tonesignals. Schmitt trigger inverter 187 provides an output waveform withfast transition times and also provides hysteresis to eliminate randomtransitions caused by noise when the signal is near the triggerthreshold point. It will be understood that the values of component 181,182, and 185 are not critical but must be selected so that the currentrating of transistor 178-b is not exceeded, rotary pulses and ringingsignals are passed, and higher frequency audio and tone signals are notpassed. The signal on conductor 111 will therefore be a digital signalwith a waveform corresponding to the shape of the envelope of the rotarypulse or ringing signals.

The output of inverter 187 is also connected by conductor 111 to thepositive trigger input of one-shot 188. One-shot 188 is a precisionretriggerable monostable multivibrator such as the Motorola MC 14538B orthe RCA 4538. The negative trigger input and the negated reset input ofone-shot 188 are connected to the system logic power supply VL byconductor 189. One-shot 188 places its Q output on conductor 112 whichis connected to one of the data inputs of latch 109-1. The Cx input ofone-shot 188 is connected by conductor 194 to the system ground and toone end of timing capacitor 193. The RxCx input is connected byconductor 192 to the other end of timing capacitor 193 and to one end oftiming resistor 191. The other end of timing resistor 191 is connectedby conductor 190 other system logic supply VL.

Resistor 191 has a nominal value of 22.1 Kohms, ±1%, and capacitor 193has a nominal value of 1.5 microfarads, ±10%, so that the period of theoutput pulse on line 112, if one-shot 188 is not retriggered, isapproximately 33 milliseconds. As will be known by those skilled in theart, a typical ringing signal comprises repeated two-second bursts of a18 Hz to 33 Hz signal, and a typical rotary dial signal comprises aseries of pulses, the rising and falling edge of each pulse beingtypically separated by 0.06 second. It will therefore be appreciatedthat capacitor 170 and bridge rectifier 172 causes inverter 187 to placea pulse onto conductor 111 for each half-cycle of the 20 Hz nominalringing signal and for both the rising and falling edges of each pulseof the rotary dial signal, thereby effectively doubling the frequency ofthese signals.

Since one-shot 188 has an unretriggered output pulse width of thirtythree milliseconds it will be appreciated that the ringing signal willkeep one-shot 188 retriggered for the entire duration of each two-secondburst. It will also be appreciated that the rotary dial signal will notkeep one-shot 188 retriggered so one-shot 188 will place on conductor112 a series of thirty-three millisecond pulses. Control unit 11therefore distinguishes a ringing signal from a rotary dial signal bymeasuring the duration of the output of one-shot 188.

Control unit 11 also distinguishes ringing signals and rotary dialsignals from noise impulses ("glitches") by measuring the period betweenoutput pulses placed on conductor 111 by inverter 187. It will beappreciated that the combination of capacitor 170 and bridge rectifier172 causes the LED/178-a of optical coupler 178 to flash twice for eachglitch, once for the leading edge of the glitch and once for thetrailing edge, thereby causing inverter 187 to place two output pulseson conductor 111. It will further be appreciated that, by definition, aglitch is of a very short duration so the two output pulses on conductor111 will be spaced closely together. However, the output pulses onconductor 111 for a rotary dial signal will be separated by asubstantially longer period of time, typically 0.07 second. A glitchwill therefore cause two closely-spaced narrow pulses on conductor 111and a slightly extended thirty-three millisecond pulse on conductor 112.

A rotary dial signal will cause, for each rotary dial pulse, two narrowpulses, separated by approximately 0.06 second, on conductor 111, andtwo thirty-three millisecond pulses, their leading edges separated byapproximately 0.06 second, on conductor 112. Each burst of the ringingsignal will cause a two-second series of output pulses on conductor 111,and a single output pulse, with a duration of slightly over two seconds,on conductor 112.

The "reminder" ringing signal when the call forwarding feature isactivated is typically a single, one-quarter burst of a ringing signal.A remainder ringing signal will therefore cause a one-half second seriesof pulse on conductor 111 and a single, approximately one-half second,pulse on conductor 112. By measuring the duration and repetition rate ofthe pulses on conductors 111 and 112 control unit 11 can distinguishbetween a glitch, a rotary dial signal, a ringing signal, and a reminderringing signal. Methods of measuring the duration and repetition rate ofthe output pulses are well known to those skilled in the art and are notdetailed herein.

FIG. 4 is a schematic diagram of isolation amplifier 113 (113-1 and113-2). Isolation amplifier 113-1 isolates and buffers signals onsubscriber line 51, and amplifier 113-2 isolates and buffers signals onextension line 47. The inverting input of high-gain amplifier 210 isconnected to conductor 101 (127) by the series combination of capacitor200 and resistor 202, to the output of amplifier 210 on conductor 114(151) by negative feedback resistor 211, and to the non-inverting inputof amplifier 210 by two reverse-oriented diodes, 208 and 209. Thenon-inverting input of amplifier 210 is connected to conductor 102 (138)by the series combination of capacitor 203 and resistor 205, to avoltage reference of one-half of the system analog power supply voltageVA by resistor 212, and to the inverting input of amplifier 210 by diode208 and 209, as described above. Capacitors 200 and 203 are D.C.blocking capacitors. Resistors 202, 205, 211 and 212 set the gain ofamplifier 210, and also set the no-input-signal output voltage ofamplifier 210 at approximately one-half of the system analog powersupply voltage. Diodes 208 and 209 protect the inputs of amplifier 210by limiting the voltage between these inputs. The output of isolationamplifier 113-1 is connected to the input of speech detector 115 and tosignal bus 35 by conductor 114. The output of isolation amplifier 113-2is connected to signal bus 35 by conductor 151. The principles ofoperation of isolation amplifier 113 are well known.

The input of speech detector 115 is connected to the output of isolationamplifier 113-1 by conductor 114. The outputs of speech detector 115 areconnected to the appropriate data inputs of output latch 109-1 byconductors 116 and 117. The operation of speech detector 115 isdiscussed in detail in U.S. Pat. No. 4,477,698 which is assigned to theassignee of the present invention.

Turn now to FIG. 5 which is a schematic diagram of DC Feed and off-hookdetector 155. Off-hook detector 155 places its output on conductor 153to indicate whether termination device 43 is on-hook or off-hook.Conductor 153 is connected to one of the data inputs of latch 109-1.Conductor 154 connects one of the data outputs of latch 108 to the gateof insulated gate field effect transistor 220. The source and substrateof transistor 220 are connected to system ground by conductor 221. Thebase of PNP transistor 228 is connected to the drain of transistor 220by resistor 223, and to the system analog power supply VA by resistor225. The emitter of transistor 228 is connected to the system analogpower supply by resistor 226. Those skilled in the art will recognizethe combination of resistors 223, 225 and 226 and transistor 228 as aconstant-current source with its output as the collector of transistor228 and will also recognize that this constant-current source is turnedon or off by transistor 220, which is turned on or off by electricalsignals on conductor 154. The collector of transistor 228 is connectedto one end of resistor 230 by conductor 229. The other end of resistor230 is connected to the anode of diode 232, to one end of resistor 235,and to the non-inverting input of open-collector comparator 236 byconductor 231. The other end of resistor 235 is connected to the systemanalog power supply VA by conductor 234. The cathode of diode 232 isconnected by conductor 152 to conductor 127 of extension line 47. Theinverting input of comparator 236 is connected to a reference voltage(VRF1) by conductor 237. In the disclosed device 10 voltage VRF1 is 10volts. The output of comparator 236 is connected to one end of a pull-upresistor 240, and to one of the data inputs of latch 109-1 of FIG. 2 byconductor 153.

Assume first that the contacts of relay 130 are closed, that transistors220 and 228 are turned off, and that central office 50 of FIG. 1 hasplaced battery voltage on subscriber line 51. If termination device 43is on-hook no current will flow through it and the voltage on conductor127 will be the central office 50 battery voltage, which is greater thanthe system analog power supply voltage VA. Diode 232 will therefore bereverse biased and resistor 235 will pull conductor 231 up to the systemanalog power supply voltage VA. Voltage VA is greater than referencevoltage VRF1 so the output of comparator 236 will be open and conductor153 will be pulled to the logic power supply voltage VL, which is alogical 1, by resistor 240 thereby indicating that termination device 43is on-hook.

Alternatively assume now that the contacts of relay 130 are open andthat transistors 220 and 228 are turned on. If termination device 43 ison-hook no current will flow through it and the voltage on conductors127 and 231 will approximate the system analog power supply voltage VA.As previously stated, voltage VA is greater than voltage VRF1 so theoutput of comparator 26 will be open, and conductor 153 will be pulledto a logical 1 by resistor 240, thereby indicating that terminationdevice 43 is on-hook.

In either case, whenever termination device 43 now goes off-hook it willsink a sufficient amount of current to cause the voltage on conductor127 to fall to a value under 9 volts. Diode 232 will now be forwardbiased so the voltage on conductor 231 will fall to a value equal to thevoltage on conductor 127 plus the forward voltage drop of diode 232. Thevoltage on conductor 231 will no be less than VRF1. Comparator 236 willthen pull conductor 153 down to system ground, which is a logical 0,thereby indicating that termination device 43 is off-hook.

It will be recalled that, if the first digit dialed from terminationdevice 43 after it goes off-hook is a * or a #, then control unit 11will cause relay 130 to open, thereby disconnecting termination device43 from operating power supplied by the central office 50. In this casecontrol unit 11 will cause a logical 1 to be placed on conductor 152,thereby turning on transistors 220 and 228, and providing operatingpower to termination device 43 from the system analog power supply.

Turn now to FIG. 6 which is a schematic diagram of loop currentsink/modulator 120 and hang-up detector 124. One of the data outputs oflatch 108 is placed onto conductor 121, which is connected to the baseof transistor 256 through the series combination of resistor 254 andconductor 255. Resistor 254 limits the base current of transistor 256 toa predetermined safe value. The emitter of transistor 256 is connectedto conductor 138. The base of transistor 261 is connected by conductor253 to conductor 138 through resistor 257, to the analog power supplyvoltage (VA) through resistor 260, to the collector of transistor 256,and to conductor 122 through the series combination of capacitor 252,conductor 251, and resistor 250. Conductor 122 is one of the conductorsof signal bus 35. The collector of transistor 261 is connected to theanalog power supply voltage (VA) by conductor 262. The emitter oftransistor 261 is connected to the base of transistor 263 by conductor258. The collector of transistor 263 is connected to conductor 128, andthe emitter is connected to conductor 138 through the series combinationof conductor 123, resistor 264, conductor 265 and potentiometer 266. Itwill be recognized that transistors 261 and 263 comprise a Darlingtonemitter follower circuit.

If conductor 121 has a logical 1 voltage on it, representing a signal torelease subscriber line 51, transistor 256 will be turned on (placed ina conducting, saturated state), effectively shorting together conductors253 and 138, which turns off transistors 261 and 263, thereby presentingan open circuit between conductors 128 and 138. It will be appreciatedthat central office 50 interprets an open circuit as an on-hookcondition. A logical 1 on conductor 121 therefore hangs up subscriberline 51.

Conversely, if conductor 121 has a logical 0 voltage on it, representinga signal to seize and hold subscriber line 51, transistor 256 will beturned off and conductor 253 will rise to a voltage (VB) determined byresistors 257 and 260. This predetermined voltage (VF) biasestransistors 261 and 263 in a linear operating region. Transistor 263therefore allows current to flow from conductor 128 to conductor 138.The amount of current flowing is determined by the voltage on conductor253 and by resistor 264 and potentiometer 266. Potentiometer 266 isadjusted so that the amount of current flowing from conductor 128 toconductor 138 is sufficient to cause central office 50 to detect an"off-hook" condition. Resistor 264 limits the current flowing throughtransistor 263 to a safe value in the event that potentiometer 266 isturned too far toward the zero-resistance position.

The purpose of resistor 250, capacitor 251, and the linear regionoperation of transistors 261 and 263 will now be explained. When thesignal on conductor 121 is a logical 0 voltage, conductor 253 rises tovoltage VB. If an audio signal is now placed on conductor 122, voltageVB will be modulated by the audio signal, therefore modulating thecurrent flowing from conductor 128 to conductor 138 through transistor263, and thereby placing the audio signal on subscriber line 51.Capacitor 252 prevents DC voltages on conductor 122 from biasingtransistors 261 and 263 into a non-linear operating region. Resistor 250protects the source which drives conductor 122 in the event thattransistor 256 is turned on. It will now be appreciated that loopcurrent sink/modulator 120 can be commanded by signals on conductor 121to seize and hold or to release subscriber line 51, and that audiosignals on conductor 122 appear as audio signals on subscriber line 51.

Referring to FIG. 1, it will also be appreciated that control unit 11causes the outputs of tone encoder 15, voice synthesizer 16, andcassette tape player 41 to be placed on subscriber line 51 by causingtheir outputs to be placed, via switching matrix 21, onto conductor 122.It will also be appreciated that rotary dialing is accomplished byplacing a series of logical 0 and logical 1 signals on conductor 121 sothat transistor 263 alternately conducts/does not conduct current fromconductor 128 to conductor 138, thereby performing the make/breakfunction required for rotary dialing.

Returning to FIG. 6, the operation of hangup detector 124 will beexplained. The emitter of transistor 263 is connected by the seriescombination of conductor 123, resistor 270, and conductor 271 to theinverting input of open collector comparator 273. The non-invertinginput of comparator 271 is connected by conductor 272 to a referencevoltage, VRF3. If loop current sink/modulator 120 has been commanded bycontrol unit 11 to seize/hold subscriber line 51, transistor 263 will beconducting current from conductor 128 to conductor 138 through resistor264 and potentiometer 266, thereby causing a voltage drop (Vc) acrossresistor 264 and potentiometer 266. This voltage (Vc) will appear onconductor 123 and, being greater than voltage VRF3, will causecomparator 273 to pull conductor 274 down to the system ground.

If termination device 54 in FIG. 1 hangs up, central office 50 willmomentarily disconnect subscriber line 51, thereby interrupting thecurrent through transistor 263, resistor 264, and potentiometer 266.This causes voltage Vc on conductor 123 to fall below VRF3, and causesthe output of open-collector comparator 273 to become an open circuitand release conductor 274. Conductor 274 connects the output ofcomparator 273 to one end of resistor 275. The other end of resistor 275is connected by conductor 276 to the system logic supply (VL) throughresistor 277 and conductor 278, to the system ground through capacitor280 and conductor 182, and to the intput of schmitt inverter 282. Theoutput of inverter 282 is placed on conductor 126.

It will be appreciated that when the output of comparator 273 is openresistor 277 pulls conductor 276 up to a logical 1, thereby causinginverter 282 to place a logical 0 on conductor 126. However, when theoutput of comparator 273 is the system ground, conductor 276 is pulleddown to a logical 0, capacitor 280 is discharged through the outputtransistor of comparator 273 and the output of inverter 282 is alogical 1. Capacitor 280 and resistors 275 and 277 form a low-passfilter and are selected so that audio signals and rotary dial pulses donot cause inverter 282 to change states. The value for resistor 275 isalso selected so as to prevent damage to the output transistor ofcomparator 273 from excessive current flow when capacitor 280 isdischarged.

It will now be appreciated that when subscriber line 51 is active andloop current sink/modulator 120 is seizing or holding subscriber line51, the voltage on conductor 123 will be greater than VRF3, the outputof comparator 273 will be the system ground, and the output of inverter282 will be a logical 1. Conversely, when termination device 54 hangsup, central office 50 will interrupt the current flow in subscriber line51, the voltage on conductor 123 will fall below VRF3, conductor 276will be pulled to a logical 1, and the output of inverter 282 will be alogical 0, thereby advising control unit 11 that the party on the otherend has hung up.

Turn now to FIG. 7 which is a schematic diagram of tone detector 161.Tone detector 161 comprises a bandpass filter 300, a high frequencydetector 334, and phase-locked loops 351 and 381. One output ofswitching matrix 21 is connected to the input of tone detector 161 byconductor 160 of signal bus 35. Control unit 11 commands switchingmatrix 21 to route the output of cassette interface 17, the signalpresent on subscriber line 51, and the signal present on extension line47, as appropriate, to tone detector 161. Tone detector 161 has fourdigital output signals: a "voice present" signal on conductor 162, a"high frequency present" signal on conductor 163, and two "tone present"signals on conductor 164 and 165. These four digital outputs areconnected to the inputs of tri-state latch 109-2 of FIG. 2 by conductors162, 163, 164 and 165. Control unit 11 reads the outputs of tonedetector 161 by commanding tri-state latch 109-2 to place its contentson data bus 25.

The tone present signal on conductor 165 may indicate the presence of an1882 Hz tone or a 1776 Hz tone. Control unit 11 commands phase-lockedloop 351 to be responsive to 1882 Hz tones by placing a logical 1 ontodata bus 25 and then commanding input latch 108 to store the signalspresent on data bus 25. Input latch 108 will then place a logical 1 ontoconductor 158, thereby causing phase-locked loop 351 to respond to 1882Hz tones. A logical 0 on conductor 158 causes phase-locked loop 351 torespond to 1776 Hz tones. Control unit 11 also commands phase-lockedloop 381 to be responsive to 3266 Hz tone or to another tone. Thefrequency of this other tone has not been designated and is reserved fora future need, if any, for another distinctive tone. Control unit 11controls phase-locked loop 381 in the same manner as it controlsphase-locked loop 351. The control input of phase-locked loop 381 isconnected to input latch 108 by conductor 159. A logical 0 on conductor159 causes phase-locked loop 381 to be responsive to 3266 Hz tones. Theoutput of phase-locked loop 381 is connected to one input of outputlatch 109-2 by conductor 164. The mechanism by which signals onconductor 158 and 159 control phase-locked loops 351 and 381 arediscussed hereinbelow.

It will be appreciated that bandpass filter 300 is an activethree-stage, multiple-pole Chebyshev-type filter which also has asecond-stage output pickoff point. The details of operation of activefilters of the Chebyshev type are well known to those skilled in theart. The first stage is a second-order lowpass filter with a gain of 4and a -3 dB frequency of 2.8 kHz. The second and third stages are secondorder highpass filters with a gain of 4.5 and a -3 dB frequency of 1kHz, and a gain of 1 and a -3 dB frequency of 2 kHz, respectively.

Since bandpass filter 300 is an active filter constructed in a knownmanner using RC networks and operational amplifiers the details ofoperation will not be discussed. However, Table I below sets forth thevalues of critical resistors and capacitors. Capacitor values are givenin microfarads and resistor values in kilohms.

                  TABLE I    ______________________________________    Reference Number                    Component Value    ______________________________________    301             3.29    303             5.6    304             .022    305             .01    307             35.5    310             11.85    314             .01    315             .01    316             9.36    317             23.77    321             10.7    322             37.46    323             .01    324             .01    325             11.45    326             4.86    330             .1    331             10.    332             1.    ______________________________________

The first two stages of bandpass filter 300 compose an active bandpassfilter which places its output on conductor 318 and which has a gain of18, and -3 dB frequencies of 1 kHz and 2.8 kHz. Normal human voices,both male and female, have a speech frequency spectrum which exhibits astrong peak in the 750 Hz region. These first two stages thereforesignificantly attenuate the primary human speech frequencies. Highfrequency detector 334, in combination with the first two stages ofbandpass filter 300, therefore detects the presence of frequencies abovethe normal primary human speech frequencies. Therefore, with theexception of the particular band of frequencies to be detected, theoperation is the same as that of speech detector 115, which is describedin detail in U.S. Pat. No. 4,477,698.

Critical resistor and capacitor values for high frequency detector 334are set forth in Table II below. Resistor values are in kilohm andcapacitor values are in microfarads.

                  TABLE II    ______________________________________    Reference Numeral                    Component Value    ______________________________________    335             4.7    336             10    337             150    340             .01    341             5.6    343             1000    345             34.0    346             1.5    ______________________________________

The three stages of bandpass filter 300 compose an active bandpassfilter with a gain of 18, a second-order high-frequency -3 dB rolloffpoint of 2.8 kHz, a second-order low-frequency -3 dB rolloff point of 1kHz, and a second, second-order low-frequency -3 dB rolloff point of 2kHz.

It should be noted that the inputs of the three stages are connected toconductor 308. Conductor 308 is connected to a voltage (1/2 VA) ofone-half of the analog power supply voltage by resistor 311. It will beappreciated that this voltage is required to bias operational amplifiers306, 320 and 327 of the three gain stages in the middle of their linearoperating region. Capacitors 312 and 313 are connected between conductor308 and the system ground. Resistor 311 and capacitors 312 and 313comprise a low pass filter which prevents noise on the system analogpower supply from affecting the voltage on conductor 308.

The output of the third stage of bandpass filter 300 is connected toconductor 308 by the series combination of capacitor 330 and resistors331 and 332. Capacitor 300 is a DC blocking capacitor which prevents theoutput of the third stage from disturbing the voltage on conductor 308.Resistors 331 and 332 comprise a voltage divider to reduce the signalvoltage being sent to phase-locked loops 351 and 381. The output ofbandpass filter 300 is on conductor 350, which is connected to thejunction of resistors 331 and 332 by DC blocking capacitor 333.

From the above, it will be appreciated that the output of bandpassfilter 300 on conductor 350 consists of that part of the signals onconductor 160 which has frequency components between 2.0 and 2.8 kHz.Conductor 350 is connected to the inputs of phase-locked loops 351 and381. Conductor 350 is connected to the input terminal of devices 362 and392 through current-limiting and parasitic suppression resistors 352 and382, respectively. Devices 362 and 392 are commercially availablephase-locked loops, such as the XR-2211, which is manufactured by ExarCompany of Sunnyvale, Calif. It will be apreciated that otherphase-locked loop devices may also be used. The frequency- andbandwidth-determining components of phase-locked loop 351 (381) arecapacitors 353 (383) and 364 (394), resistors 335 (385), 356 (386), 357(387), 360 (390), and 361 (391), and the conduction state of transistor354 (384). Resistor and capacitor values are set forth in Table IIIbelow. Resistor values are in kilohms and capacitor values are inmicrofarads.

                  TABLE III    ______________________________________    Reference Numeral                    Component Value    ______________________________________    353             .022    355             10    356             27.4    357             10    360             17.8    361             100    364             .0047    383             .01    385             10    386             1    387             10    390             24.9    391             150    394             .0022    ______________________________________

Capacitor 353 (383) is connected between the C13 and C14 terminals ofdevice 263 (392). The Loop Phase-Detector output of device 362 (392) isconnected to system ground through capacitor 364 (394) and to theR-input of device 362 (392) through resistor 361 (391). The R-input ofdevice 362 (392) is connected to node 358 (388) through resistor 360(390). Node 358 (388) is connected to system ground through resistor 357(387) and also through the series combination of resistors 355 and 356(385 and 386) and field effect transistor 354 (384). If conductor 158(159) has a logical 0 placed on it, transistor 354 (384) will be turnedoff and appear as an open circuit. The frequency of phase-locked loop351 (381) will therefore be determined by capacitor 353 (383) and theseries resistance of resistors 357 and 360 (387 and 390). Resistor 357(387) is a trimmer potentiometer and is adjusted to set the frequency ofloop 351 (381) to 1776 kHz (3266 Hz).

If the voltage on conductor 158 (159) is a logical 1 transistor 354(384) will be turned on and appear as a short circuit. The frequency ofphase-locked loop 351 (381) will then be determined by capacitor 353(383) and the effective resistance of resistor 360 (390) in series withresistor 357 (387) which is in parallel with the series combination ofresistors 355 and 356 (385 and 386). The effectivd resistance willtherefore be lower and the detected frequency, which is inverselyproportional to the effective resistance, will be higher. Resistor 355is a potentiometer which is adjusted to set the second tone frequency ofloop 351 to 1882 Hz. Resistor 385 performs a similar function withrespect to loop 381 but, in this particular application, the second tonefrequency is unspecified since it is not currently needed for anypurpose.

The frequencies 1882 Hz and 3266 Hz were selected to detect signalsindicating the beginning and end, respectively, of a message recorded oncassette recorder 40 and cassette player 41.

The 1776 Hz frequency tone was selected since this is the standard thirdtone of the three-tone interrupt sequence that central office 50 placeson subscriber line 51 to advise the user that the call cannot becompleted as dialed.

The negated Q output of device 362 (392) is an open-collector outputwhich is a logical 0 when there is no frequency lock, i.e., the desiredtone is not present, and is an open circuit when there is a frequencylock, i.e., the desired tone is present. The negated Q output of device362 (392) on conductor 369 (399) is connected to the logic supplyvoltage VL by pullup resistor 370 (400) and by the series combination ofcapacitor 371 (401) and resistor 372 (402), and is also connected to theinput of schmitt-trigger inverter 373 (403). The output of inverter 373(403) is connected to conductor 165 (164). Inverter 373 (403) is of theschmitt type to prevent minor power supply voltage variations and noisefrom causing an erroneous signal to be placed on conductor 165 (164).

It will now be remembered that bandpass filter 300 passes 2.0 to 2.8 kHzsignals to phase-locked loops 351 and 381. Although the primaryfrequency of normal human speech is in the region of 750 Hz it will berecognized that some components of human speech are briefly present inthe 2.0 to 2.8 kHz band. Devices 362 and 392 may briefly andintermittently lock onto these speech components, thus giving anerroneous tone-detect signal. Resistor 372 (402) and capacitor 371 (401)form a filter which requires the output of device 362 (392) to maintainan open circuit or a logical 0 output for a short period of time beforethe voltage on conductor 369 (399) will change a sufficient amount tocause inverter 373 (403) to change its output state therebysubstantially reducing the probability of an erroneous tone-detectsignal.

From the foregoing description of the preferred embodiment, it will beapparent that other embodiments of the present invention may beconstructed on the principles disclosed herein. Therefore, the scope ofthe present invention is to be limited only by the claims below.

I claim:
 1. Call forwarding control apparatus for use on a subscribertelephone line connected to a central office of the type which causes acall forwarding function to be activated in response to provision oftone signals corresponding to a first predetermined code signal followedby a destination telephone number, comprising in combination:atermination device for selectively generating tone signals correspondingto said first predetermined code signal, said destination telephonenumber, and a second predetermined code signal; off-hook detector meansconnected to said termination device for providing an off-hook signalwhen said termination device is in an off-hook condition; tone decodermeans responsive to tone signals for providing digital output signalscorresponding to each said tone signal; switching means responsive tothe provision of said off-hook signal for connecting said tone decodermeans to said termination device and, otherwise, for connecting saidtone decoder means to said subscriber line; storage means forselectively storing said digital output signals as stored digital words;tone generator means selectively responsive to said stored digital wordsfor providing to said central office over said subscriber line said tonesignals corresponding to said first predetermined code signal followedby said destination telephone number; and forwarding control means (a)responsive to a first predetermined sequence of said digital outputsignals for placing said call forwarding control apparatus in aforwarding state and for activating said call forwarding function byrendering said tone generator means responsive to said stored digitalwords thereby providing to said central office over said subscriber linesaid tone signals corresponding to said first predetermined code signalfollowed by said destination telephone number, (b) responsive toprovision of said off-hook signal and to said digital output signalscorresponding to said first predetermined code signal followed by saiddestination telephone number for placing said call forwarding apparatusin a forwarding state, and (c) responsive to provision of said off-hooksignal and to said digital output signals corresponding to said secondpredetermined code signal for placing said call forwarding apparatus ina forwarding state and for activating said call forwarding function byrendering said tone generator means responsive to said stored digitalwords thereby providing to said central office over said subscriber linetone signals corresponding to said first predetermined code signalfollowed by said destination telephone number.
 2. Apparatus as recitedin claim 1 wherein:said tone signals are dual tone multiple frequencysignals.
 3. The apparatus of claim 1 wherein said termination device isa telephone.
 4. Call forwarding control apparatus for use on asubscriber telephone line connected to a central office of the typewhich causes a call forwarding function to be deactivated in response toprovision of tone signals corresponding to a first predetermined codesignal, and of the type which provides a predetermined electrical signalto said subscriber line in response to each forwarded call when saidcall forwarding function is active, comprising in combination:atermination device for selectively generating tone signals correspondingto said first predetermined code signal and a second predetermined codesignal; off-hook detector means connected to said termination device forproviding an off-hook signal when said termination device is in anoff-hook condition; tone decoder means connected to said terminationdevice and responsive to tone signals for providing digital outputsignals corresponding to each said tone signal; counting means connectedto said subscriber line and responsive to the number of occurrences ofsaid predetermined electrical signal when said apparatus is in aforwarding state for providing a digital count signal when said numberof said occurrences is equal to a predetermined count number; storagemeans for storing said first predetermined code signal as a storeddigital word; tone generator means selectively responsive to said storeddigital word for providing to said central office over said subscriberline said tone signals corresponding to said first predetermined codesignal; and forwarding control means (a) responsive to said digitalcount signal and to the state of said call forwarding control apparatusfor placing said call forwarding control apparatus in a non-forwardingstate and for deactivating said call forwarding function by renderingsaid tone generator means responsive to said stored digital word therebyproviding to said central office over said subscriber line said tonesignals corresponding said first predetermined code signal, (b)responsive to said digital output signals corresponding to said firstpredetermined code signal and to provision of said off-hook signal forplacing said call forwarding apparatus in a non-forwarding state, and(c) responsive to said digital output signals corresponding to saidsecond predetermined code signal and to provision of said off-hooksignal and to the state of said call forwarding control apparatus forplacing said call forwarding control apparatus in a non-forwarding stateand for deactivating said call forwarding function by rendering saidtone generator means responsive to said stored digital word therebyproviding to said central office over said subscriber line said tonesignals corresponding to said first predetermined code signal. 5.Apparatus as recited in claim 4 wherein:said predetermined count numberis selectively variable.
 6. Apparatus as recited in claim 4 and furthercomprising:timing means operating when said apparatus is in saidforwarding state for causing said counting means to provide said digitalcount signal if and only if said predetermined count number ofoccurrences of said predetermined electrical signals occurs within apredetermined time window.
 7. Apparatus as recited in claim 6wherein:the period of said predetermined time window is selectivelyvariable.
 8. Apparatus as recited in claim 4 wherein:said tone signalsare dual tone multiple frequency signals.
 9. The apparatus of claim 4wherein said termination device is a telephone.
 10. Call forwardingcontrol apparatus for use on a subscriber telephone line connected to acentral office of the type which causes a call forwarding function to beactivated in response to provision of tone signals corresponding to afirst predetermined code signal followed by a destination telephonenumber, and to be deactivated in response to provision of tone signalscorresponding to a second predetermined code signal, and of the typewhich provides a predetermined electrical signal to said subscriber linein response to each forwarded call when said call forwarding function isactive, comprising in combination:a termination device for selectivelygenerating tone signals corresponding to said first predetermined codesignal, said destination telephone number, said second predeterminedcode signal, a third predetermined code signal and a fourthpredetermined code signal; off-hook detector means connected to saidtermination device for providing an off-hook signal when saidtermination device is in an off-hook condition; tone decoder meansresponsive to tone signals for providing digital output signalscorresponding to each said tone signal; storage means for storing saiddigital output signals as stored digital words; switching meansresponsive to the provision of said off-hook signal for connecting saidtone decoder means to said termination device and, otherwise, forconnecting said tone decoder means to said subscriber line; tonegenerator means selectively responsive to said stored digital words forproviding to said central office over said subscriber line tone signalscorresponding to said first and second predetermined code signals andsaid destination telephone number; counting means connected to saidsubscriber line and responsive to the number of occurrences of saidpredetermined electrical signal when said apparatus is in a forwardingstate for providing a digital count signal when said number of saidoccurrences is equal to a predetermined count number; and forwardingcontrol means (a) responsive to a first predetermined sequence of saiddigital output signals from said tone decoder means for placing saidcall forwarding control apparatus in a forwarding state and foractivating said call forwarding function by rendering said tonegenerator means responsive to said stored digital words therebyproviding to said central office over said subscriber said tone signalscorresponding to said first predetermined code signal followed by saiddestination telephone number, (b) responsive to provision of saidoff-hook signal and to said digital output signals corresponding to saidfirst predetermined code signal followed by said destination telephonenumber for placing said call forwarding apparatus in a forwarding state,(c) responsive to said provision of said off-hook signal and to saiddigital output signals corresponding to said third predetermined codesignal for placing said call forwarding control apparatus in aforwarding state and for activating said call forwarding function byrendering said tone generator means responsive to said stored digitalwords thereby providing to said central office over said subscriber linesaid tone signals corresponding to said first predetermined code signalfollowed by said destination telephone number, (d) responsive to saiddigital count signal and to the state of said call forwarding controlapparatus for placing said call forwarding control apparatus in anon-forwarding state and for deactivating said call forwarding functionby rendering said tone generator means responsive to said stored digitalwords thereby providing to said central office over said subscriber linesaid tone signals corresponding to said second predetermined codesignal, and (e) responsive to said digital output signals correspondingto said second predetermined code signal and to provision of saidoff-hook signal for placing said call forwarding apparatus in anon-forwarding state, and (f) responsive to said digital output signalscorresponding to said fourth predetermined code signal, to provision ofsaid off-hook signal and to the state of said call forwarding controlapparatus for placing said call forwarding control apparatus in anon-forwarding state and for deactivating said call forwarding functionby rendering said tone generator means responsive to said stored digitalword thereby providing to said central office over said subscriber linesaid tone signals corresponding to said second predetermined codesignal.
 11. The apparatus of claim 10 wherein said termination device isa telephone.
 12. Apparatus as recited in claim 10 wherein:saidpredetermined count number is selectively variable.
 13. Apparatus asrecited in claim 10 and further comprising:timing means operating whensaid apparatus is in said forwarding state for causing said countingmeans to provide said digital count signal if and only if saidpredetermined count number of occurrences of said predeterminedelectrical signals occurs within a predetermined time window. 14.Apparatus as recited in claim 13 wherein:the period of saidpredetermined time window is selectively variable.